Roof falls are the number one cause of coal mine injuries and fatalities. Mine roof bolts are the principal means of roof support to prevent roof falls. Millions of mechanical anchor roof bolts are used each year for this purpose. The benefit of this bolting depends on a number of factors including rock density, bolt length, in situ stresses, time elapsed between mining and installation, rock properties, bolt patterns, and tension.
Installation tension is a major determinant of support quality. Post installation tensions indicate both the usefulness of the individual bolts and the behavior of the reinforced rock structure. The importance of short and long term bolt tension measurements is well recognized and is required by regulations (30 CFR 55, 56 and 57). These regulations also take into account the difficulty of practical tension measurements. Only a statistical sample of the bolts installed are required to be measured using current torque wrench technology. Drawbacks to the torque wrench technology are that the anchorage is disturbed and perhaps weakened, accuracy is limited by friction to about + or -30 percent (+ or -3,500 lbs), and the procedure is relatively time intensive.
Many instrument concepts have been considered in the past for measuring the strain in mine roof bolts. In general, these concepts can be grouped into two categories; those concepts which provide good measurement accuracy but at high cost per bolt, and those concepts which provide low cost per bolt but limited accuracy.
One instrument concept which has been considered in the past is ultrasonic pulse-echo measurement. In this concept, a transducer is mounted on the bolt head to introduce the pulse into the bolt. The time taken to travel down the bolt and reflect back to the transducer is measured. This travel time is obviously related to the bolt length. Once the initial length is determined, the change in travel time is related to a bolt strain. From the geometry of the bolt, or from empirical calibration, this can be related to bolt tension or load.
Although the ultrasonic pulse-echo measurement concept is simple in theory and can be easily applied to the measurement of bolt strain in laboratory situations, the application of such a concept to mine roof bolts is quite complex. Among the complicating factors are the need for extreme time measurement accuracy, the ultrasonically undesirable geometry of mine roof bolts, variations in bolt lengths before installation, variations in effective bolt lengths due to anchor nut positions, stress and temperature effects on ultrasonic velocity, roughly forged bolt constructions including roughly forged heads with raised length and grade markings, the long and narrow geometry of mine roof bolts, plastic strain and signal attenuation caused by bending, portability requirements, gassy mine permissibility requirements, adverse operating environment, and the need for long term repeatability. In contrast, presently available pulse-echo instruments work well only on precision industrial bolts which have flat ends, flat heads, measurable installed effective lengths, tight dimensional tolerances, relatively short lengths, relatively large diameters, relatively mild environments, and no bending.
Various ultrasonic devices have been disclosed in the prior art for determining bolt tension or strain. For example, in U.S. Pat. No. 4,471,657 (Voris et al), an ultrasonic stress measuring method and apparatus is disclosed for measuring the length and stress in a tensile load member such as a bolt. The apparatus includes time interval measuring means for determining the elapsed time between transducer energization and the receipt of a pulse echo. Besides basing the strain on the measured time interval, a variety of other factors are also considered including temperature, tensile load member material, velocity change due to stress forces on the tensile load member material, overall length, elasticity of the tensile load member material, and thermal expansion of the tensile load member material. Other ultrasonic bolt tension measuring devices are disclosed in U.S. Pat. No. 4,062,227 (Heyman) and U.S. Pat. No. 4,402,222 (Olson et al). An ultrasonic device for measuring strain in bolts using a pulse phase lock loop technique is disclosed in U.S. Pat. No. 4,363,242 (Heyman).
Even though there has been a demonstrated need for a low cost, high accuracy method of measuring mine roof bolt strain, such a technique has not been achieved in the prior art.